We can't shed any light on how Shakespeare wrote his plays or Mozart composed his music. But, as 60 Minutes first reported last December, there is a new theory that may explain how Leonardo and the other Old Masters created their masterpieces.
Did they have help? Yes. Was it magic? No. But it wouldn't be entirely wrong to say they did it with mirrors. Correspondent Lesley Stahl reports.
You can't stand in front of the paintings of the "Old Masters" and not wonder, "How did Frans Hals make that lace seem so real? How did van Eyck make his armor so gleamingly metallic? How did Caravaggio make his faces so expressive and lifelike?"
The theory is they had help with lenses or concave mirrors. If someone stands outside bathed in light, an image can be projected inside onto a wall, upside down. That projected image can then be copied.
David Hockney, one of our best-known artists, believes that the Old Masters used the early technology of optics and kept it secret.
"I'm suggesting that artists saw these projections," he says. "They're very simple to make, and when you make them, they're very beautiful and exciting."
When set up carefully in a studio, the projection is bright and clear. To demonstrate how Caravaggio might have painted his picture of Bacchus, Hockney, in a film he made for the BBC, arranged a model and projected his image into a dark room - what artists call a camera obscura.
The image was cast directly onto his canvas, and he traced it. It was so much easier than painting from life. Once the artists saw these flattened-out two-dimensional projections, says Hockney, they couldn't resist.
"It's hard to believe that in the 15th century they would say, 'What an amusing novelty, how interesting ... but let's not use that,'" he says.
Hockney comes at this with a practiced eye. Over the last four decades, he's established himself as one of the leading contemporary artists with his drawings, his set designs, his photo collages and of course, his paintings.
But the man who made icons out of Hollywood swimming pools never imagined that he'd be jumping into his own pool of controversy with his new take on the Old Masters.
Once Hockney figured out how the pictures were made, he set out to discover where and when the use of optics began. In his studio in Los Angeles, he built his great wall of hundreds of paintings spanning hundreds of years.
"We did come to about 1420, and realized something happens," he says.
What happens is a sudden appearance of realism. Before 1420, faces were idealized; immediately after, they were true to life. Before, garments were flat and formless. After, they were vivid and photographic.
Hockney says it started in Bruges, Belgium, one of Europe's great 15th century commercial centers, where that optical look, a photographic look, first appeared in the works of Flemish masters like Jan van Eyck.
"[He was] a painter who knew about optical projections and had looked at them," says Hockney. "One thing the mirror projections do is project surfaces quite amazingly, especially shiny surfaces. And there's lots of shiny surfaces."
What's so revolutionary about what he is saying? The history of art - the history of the Renaissance - is the history of optics.
Needless to say, Hockney and his book about this, called "Secret Knowledge," have rocked the art world, where most art historians say it's bunk.
"All these art historians, not one of them ever took the trouble to look through a camera obscura to see what it was like," says Hockney.
They don't like the idea that, as Hockney suggests, the Old Masters traced their creations. There is an implication of cheating in that.
But Hockney says they weren't cheaters, but great innovators: "Not only did they have skills you think you know, they had marvelous skills about optical things as well."
Hockney points to van Eyck's "Arnolfini Wedding." He used to wonder how did the painter paint the chandelier in the picture: "That chandelier is in perfect perspective. So how was it drawn?" He now believes it was created with a concave mirror, and a pencil.
Walter Liedtke, curator of European paintings at New York's Metropolitan Museum of Art, disagrees with Hockney's contention that the Renaissance started in Bruges and Florence because of optics.
"Isn't there something about those cultures, the fact that they're predominantly urban, mercantile, sophisticated, with a strong middle class," says Liedtke.
Hockney, however, has debated the cultural explanation with Liedtke and other art historians.
Critics point out that there's little mention of optics or tracing in the historical record, nothing in documents or diaries.
"Yes, but the historical record has no mention of how they painted the pictures either," says Hockney in response. "The only people who wrote in diaries were highly educated."
Even today, he says, the artists wouldn't tell: "They're very secretive. Remember, they're competing in business as well."
It was also the time of the Inquisition, when mirrors and lenses were associated with witchcraft.
"When Caravaggio is painting in Rome, around the corner in the square, they're burning Claudio Bruni for looking through lenses," says Hockney.
Hockney is amazed that art historians don't see what he thinks is obvious - the optical evidence right there in the works of such formidable artists as Holbein, Velazquez, and even Leonardo de Vinci.
"Leonardo describes the camera obscura, meaning he tried it out and looked at the pictures," he says.
Hockney isn't saying Leonardo traced the "Mona Lisa," but that it has the qualities you see in an optical projection, like the soft shadows.
"Whether he used the lens for this, I don't know. Nobody knows. It wouldn't matter. He wouldn't need it, but he'd already seen the wonderful softness," he says.
Then there are all those left-handed people who suddenly showed up in paintings - Hockney's smoking gun. If you just use a lens alone, left becomes right, and vice-versa.
Dr. Charles Falco, a physicist at the University of Arizona, and an expert in optics, heard about Hockney's theory and was fascinated.
As a scientist, he thought, "If this is true, I can prove it." The first painting that caught his eye was the 1543 "Wedding Portrait" by Lorenzo Lotto.
"He made a mistake. That's what told me a lens was used. This central pattern of this geometrical tablecloth goes out of focus," says Falco. "Your eye doesn't naturally see something out of focus. The only way you could see this feature is if you'd seen something with a lens."
He also found more evidence in the portrait of Cardinal Albergati by van Eyck: "This is one of the rare examples where a preliminary drawing exists along with the painting. I looked at this and said, 'My God, this is good enough to be a photocopy.'"
"So let's see how actually perfect it is," he adds. "I'll blow up the drawing to the same scale as the painting, and when I overlay them, every feature, every wrinkle, every hair is accurate."
Except for the ear: "It's completely off. What if he bumped his easel when he's doing this? Watch the ear. It's perfect."
But Liedtke says there are other explanations. For example, he told us van Eyck could have used a geometric grid to enlarge his drawings.
"I think it's simply been taken too far," says Liedtke. "We now have a kind of optical explanation for realism in Western art. And the, I have to say, the celebrity factor here is really important. If I wrote this book and submitted it to a publisher, it would be sent back to me. But it's a little bit like Jane Fonda's workout or Steven Seagal's Buddhism. You know? It's David Hockney's optics, and so it goes down well."
He believes that it's remotely possibly that this could have happened, and it probably happened in some cases, but "simply not to the extent that it rewrites the history of art."
Hockney's optics is a rewriting of the history of art. But he insists it in no way diminishes the artists.
"Actually, my respect for them is more," he says. "If you were given a tracing of a Vermeer, 'Here, now you paint the Vermeer.' Absurd to think that 'Ah, well, that's done and now I can do it.' You can't."